Well, the option is ticked. However, interestingly, when I tried to disable it, it needed two clicks to confirm. Then, when I ticked the option to enable it, the light and spawning points can be seen now.

However, monsters are still not being shown for whatever reason, and I tested by adding another enemy to the map.

Not sure if relevant, but I have noticed that after loading the map, the textures area in the editor shows textures found in the map itself. For example, I see the textures of the imp, LMS marine, doom 3 icons, loading screens and whatever else. I don't think it ever did that, but I could be wrong.

Perhaps some texture setting is enabled?

If anybody has any idea on what might bring the monsters back, it would be really appreciated.